Ligand-directed top-down synthesis of trivacant lacunary polyoxomolybdates from plenary Keggin-type [α-XMo12O40]3− (X = P, As, V) in organic media

Abstract

Lacunary polyoxometalates (POMs), featuring highly reactive vacant sites, serve as valuable building blocks and precursors for the rational design of functional materials with widespread applications in catalysis, analytical chemistry, energy conversion and storage, medicine, and optical materials. While diverse Keggin-type polyoxotungstates, including both plenary and lacunary species, have been synthesized through dehydration condensation reactions and equilibrium displacement in aqueous solvents, the isolation and use of lacunary polyoxomolybdates remain challenging. To address this, the current study proposes a “ligand-directed top-down synthetic approach” for producing lacunary polyoxomolybdates from plenary Keggin-type species in organic solvents. By reacting plenary Keggin-type polyoxomolybdates [α-XMo₁₂O₄₀]³⁻ (X = P, As, V) with 4-methoxypyridine (pyOMe) in acetonitrile, we successfully synthesize the corresponding trivacant lacunary polyoxomolybdates ([XMo₉O₃₁(pyOMe)₃]³⁻), where the vacant sites are stabilized by three pyOMe ligands. Remarkably, this approach enables the synthesis of a lacunary vanadomolybdate, a species previously unattainable through equilibrium control in aqueous systems. Furthermore, the reversible coordination of pyOMe ligands to molybdenum atoms at the vacant sites makes these lacunary polyoxomolybdates highly versatile precursors for assembling POM–organic hybrids. Overall, this study introduces an innovative synthetic methodology for POMs, demonstrating notable potential for advancing the development of functional materials.